The present invention relates to a zoom lens for an optical system of a copying machine or the like, and more particularly to a three-unit type copying zoom lens having a F-number of approximately 1:7 and enabling to covering a half view angle of approximately 20.degree., in which a distance between the object and the image may be kept constant.
Recently, there have been strong demands that copying lenses be miniaturized are made at low cost as copying machines have been made compact at low cost. Also, there are like needs for zoom lenses that are used for enlarging and reducing operation in the copying machines.
A three-unit copying lens is disclosed in Japanese Patent Application Laid-Open Nos. 57-67909 and 60-121414. However, such a zoom lens needs 8 to 10 lens elements, resulting in high cost. Thus, the zoom lens might not meet the recent requirement that a supercompact copying machine be made at low manufacture cost.
Another copying lens is proposed in Japanese Patent Application Laid-Open No. 61-140912 in order to meet the low cost requirement. However, this lens is composed of seven elements grouped into seven unit, and is of a wide angle type covering a half view angle of 25.degree.. Therefore, this lens has a relatively large number of structural elements for the half view angle of about 20.degree., which could not attain the low cost requirement. This should be further improved.